The present invention relates to a fixing device for a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to an induction heating type of fixing device.
An induction heating type of fixing device for use in an image forming apparatus is configured to heat the wall or core of a heat roller with Joule heat derived from induced current. Specifically, this type of fixing device includes electromagnetic induction heating means having an induction heating coil. High frequency current is fed to the induction heating coil to cause it to generate an induced flux, which in turn generates induced current (eddy current) in a conductive layer covering the heat roller. Joule heat derived from the induced current heats the surface of the heat roller to a preselected temperature. It is a common practice to produce the high frequency current by rectifying AC available with a commercial power source with a rectifying circuit and then converting it to high frequency.
A conventional inverter circuit for induction heating stabilizes the fixing temperature of the fixing device by varying frequency. A problem with this conventional scheme is that the varying frequency translates into the variation of the penetration depth of the eddy current and thereby prevents power for maintaining optimal fixing temperature from being input to the heat roller. Further, the variation of the penetration depth of the eddy current causes the heat distribution on the surface of the heat roller to vary, effecting the quality of a fixed image.
When the inverter circuit is configured for an AC 200 V application, it needs a switching device that withstands voltage two times as high as the withstanding voltage of a switching device for an AC 100 V application. A switching device for an AC 200 V application and comparable in size with a switching device for an AC 100 V application is rare or is insufficient in withstanding voltage if available. While a mold type switching device withstands high voltage, it is packaged in a size more than two times as great as the size of a 100 V switching device. This kind of switching device is not applicable to a high frequency inverter for use in a fixing device. It has therefore been difficult to realize a miniature inverter circuit adaptive to a 200 V application.
Moreover, a power control range available with the conventional inverter circuit is narrow. Therefore, when the load of the inverter circuit is light, current flowing through the induction heating coil or work coil is short and prevents current from being fully discharged from a resonance capacitor. It follows that the inverter circuit fails to perform zero voltage switching and looses its high efficiency and low noise features based on zero voltage switching.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 9-245953 and 2000-259018.
It is an object of the present invention to provide a fixing device using an inverter circuit for induction heating that achieves high efficiency and reduces the stress of a switching device as well as switching noise. In accordance with the present invention, an inverter circuit for induction heating includes a switching device that drives one end of an induction heating coil the other end of which is connected to a power source. A capacitor and a second switching device are serially connected to each other and connected to opposite ends of the induction heating coil in parallel such that one end of the capacitor is connected to the power source. A second capacitor is connected to the second switching device in parallel. The second capacitor has a capacitance of 0.1 xcexcF to 0.4 xcexcF. For a capacitance of 0.1 xcexcF of the second capacitor, the induction heating coil has an inductance of 70 xcexcH to 100 xcexcH while the capacitor has a capacitance of 1.8 xcexcF to 5 xcexcF. Also, for a capacitance of 0.2 xcexcF of the second capacitor, the induction heating coil has an inductance of 65 xcexcH to 100 xcexcH while the capacitor has a capacitance of 1.8 xcexcF to 5 xcexcF. Further, for a capacitance of 0.3 xcexcF of the second capacitor, the induction heating coil has an inductance of 65 xcexcH to 95 xcexcH while the capacitor has a capacitance of 2 F to 5 F. Moreover, for a capacitance of 0.4 xcexcF of the second capacitor, the induction heating coil has an inductance of 65 xcexcH to 87 xcexcH while the capacitor has a capacitance of 2.3 xcexcF to 5 xcexcF.